(1) Field of the Invention
This invention relates to X-ray CT apparatus for enabling what is known as CT fluoroscopy.
(2) Description of the Related Art
With an X-ray CT apparatus of this type, an image reconstructing process can be carried out in a shorter time than an image reconstruction performed at a so-called off time. By using the image reconstructing process carried out in such a short time, numerous X-ray CT images may be picked up successively in real time and at a relatively fast rate such as 8 frames per second, for example. Thus, what is known as CT fluoroscopy is performed for observing X-ray CT images, which are sectional images, in the form of continuous dynamic images as in usual X-ray fluoroscopy. CT fluoroscopy, as it enables a real-time observation of continuous images, is used mainly in catheterization such as biopsy. Biopsy and the like usually take a relatively long time, i.e. from several tens of seconds to several minutes.
However, CT fluoroscopy cannot be performed continuously for a long time with a conventional X-ray CT apparatus capable of CT fluoroscopy. That is, with the conventional apparatus, while successively storing in memory all projection data acquired from CT fluoroscopy, an image is reconstructed each time projection data for predetermined angles enabling an image reconstruction has been collected, and such images are displayed successively. Thus, the time for continuously performing CT fluoroscopy is always limited by a storage capacity of memory for storing the projection data.
In time of CT fluoroscopy, numerous images are observed continuously. In practice, it often is the case that only several of those images are useful. In such a case, even though all the projection data collected are stored as with the conventional apparatus, only a small part of the data is reused (e.g. for reconstructing and observing images after CT fluoroscopy). This amounts to a wasteful consumption of memory.
This invention has been made having regard to the state of the art noted above, and its object is to provide an X-ray CT apparatus which uses memory efficiently to enable CT fluoroscopy to be performed continuously over an extended time, and allows projection data to be saved as necessary.
The above object is fulfilled, according to this invention, by an X-ray CT apparatus for enabling CT fluoroscopy comprising:
a scanning device including an X-ray tube and an X-ray detector for continuously repeating an X-ray scan to collect X-ray projection data from directions of varied angles;
a first storage having a storage capacity for storing projection data for angles necessary to reconstruct at least one image, the first storage storing, after the projection data for the angles is written therein, new projection data as written over the projection data written earliest;
a second storage having a storage capacity for storing projection data for angles necessary to reconstruct numerous images, the second storage successively storing projection data successively collected, without rewriting the projection data, until the storage capacity is exhausted;
an image reconstructing device for reconstructing an image from projection data for predetermined angles;
a control device for controlling storage of the projection data successively collected in a selected one of the first and second storages and transmission of the projection data to the image reconstructing device; and
a display device for displaying the image reconstructed by the image reconstructing device.
According to this invention, when a control is effected to store projection data successively collected in the first storage, the first storage stores the latest projection data for angles necessary to reconstruct at least one image. CT fluoroscopy may be performed by reconstructing images from this data at high speed, and displaying the latest X-ray CT images thereby successively acquired on the display device. In this case, since the first storage is constantly updated with a predetermined amount of latest projection data, CT fluoroscopy may be performed continuously over an extended time without a limitation by the storage capacity.
When a control is effected to store projection data successively collected in the second storage, the second storage, as does a storage in a conventional X-ray CT apparatus, successively stores, in the order of collection, the projection data successively collected until the second storage becomes full. The data collection therefore is limited by its capacity, but the data may be read again for image reconstruction to enable observation of selected images.
Thus, according to this invention, CT fluoroscopy may be performed continuously without a limitation of time, or, though limited in time, the projection data acquired for CT fluoroscopy may be saved for use in reproducing, with high image quality afterward, the images seen in time of CT fluoroscopy.
The above X-ray CT apparatus may further comprise a selection instructing device for instructing a selection of one of the first and second storages for storing the projection data collected, and transmission of the projection data to the image reconstructing device. The control device may perform controls, on instructions from the selection instructing device, to determine which of the first and second storages should be used for storing the projection data, and whether the projection data should be transmitted to the image reconstructing device.
This construction allows the operator to select and instruct which of the first and second storages the projection data should be stored in.
In the above X-ray CT apparatus, the control device may perform CT fluoroscopy by determining beforehand which of the first and second storages should be used for storing the projection data collected during the CT fluoroscopy. This selection may be instructed from the selection instructing device.
With this construction, a procedure of CT fluoroscopy not requiring the projection data to be saved may be carried out continuously without a limitation of time while storing and updating projection data only in the first storage. On the other hand, in a procedure of CT fluoroscopy requiring all the projection data to be saved, the projection data is successively stored, without overwriting, only in the second storage, thereby saving all the projection data collected in time of CT fluoroscopy, to enable a subsequent reproduction, with high image quality, of the images observed in time of CT fluoroscopy.
In the above X-ray CT apparatus, the control device may perform CT fluoroscopy while switching between the first and second storages for storing the projection data collected during the CT fluoroscopy.
With this construction, during CT fluoroscopy, projection data not to be saved may be stored in the first storage, while only projection data to be saved may be stored in the second storage. Thus, a continuous procedure of CT fluoroscopy may be performed for an extended time. In addition, among the projection data collected during one cycle of CT fluoroscopy, only necessary data may be saved for later use. In this way, the memory may be used efficiently.
In the above X-ray CT apparatus, the selection instructing device may be operable to instruct, during CT fluoroscopy, switching between the first and second storages for storing the projection data collected during the CT fluoroscopy. This switching of the storages may be effected as follows.
The control device is operable to store the projection data collected during CT fluoroscopy normally in the first storage, to switch storage of the projection data from the first storage to the second storage when an instruction is given from the selection instructing device to select the second storage for storing the projection data, and to switch storage of the projection data from the second storage to the first storage upon lapse of a predetermined time from the instruction.
With this construction, the operator may input a storage switching instruction at a point of time when it becomes necessary to save the projection data during CT fluoroscopy. Then, the projection data collected over a fixed period from the selected point of time may be saved for later use.
The control device may be operable to switch storage of the projection data always upon selecting instructions from the selection instructing device. That is, the control device relies solely on selecting instructions from the selection instructing device for switching from the first storage to the second storage and vice versa for storing the projection data during CT fluoroscopy.
With this construction, the operator may input, at a point of time when it becomes necessary to save projection data during CT fluoroscopy, an instruction to switch from the first storage to the second storage. When it is no longer necessary to save the projection data, the operator may input an instruction to switch from the second storage to the first storage. Thus, the projection data collected over a period between points of time selected by the operator may be saved for later use.
The above X-ray CT apparatus may further comprise a copying device for copying projection data from the first storage to the second storage.
It will serve the purpose as long as the projection data to be copied includes the latest projection data for angles necessary to reconstruct at least one image. Thus, the projection data to be copied may be all the projection data currently stored in the first storage, or only the latest projection data for angles necessary to reconstruct one image.
The copying device may be operable, when X-ray radiation from the X-ray tube is discontinued, with projection data collected before discontinuation of the X-ray radiation being stored in the first storage, to copy the projection data from the first storage to the second storage.
A procedure of CT fluoroscopy could be suspended by temporarily stopping the X-ray radiation from the X-ray tube. With the above construction, even if the projection data collected before the suspension is stored in the first storage, the projection data may be copied automatically to the second storage to be saved for use in reconstructing an image acquired immediately before the suspension.
The X-ray CT apparatus having the copying device may further comprise a copy instructing device for instructing copying of the projection data, the copying device being operable, upon an instruction from the copy instructing device, to copy the projection data from the first storage to the second storage.
With this construction, when the operator decides to save the projection data, though stored in the first storage, the projection data may be copied to the second storage to be saved for later use.
The copying device may be operable to copy the projection data from the first storage to the second storage whenever the control device switches storage of the projection data from the first storage to the second storage during CT fluoroscopy.
Where, for example, a data storage switching is made from the first storage to the second storage upon an instruction from the selection instructing device, the operator gives this switching instruction while observing an image displayed on the display device. It is the first storage that stores the projection data determined by the operator to be necessary for reconstructing the image then displayed on the display device. Thus, the projection data could be lost. With the above construction, however, the projection data stored in the first storage is copied to the second storage also to be saved therein, immediately before the control device switches from the first storage to the second storage for storing the projection data during CT fluoroscopy.
The first and second storages may have separate areas in a memory allocated thereto for storing the projection data.
This feature simplifies controls for storing projection data in the memory.
The first and second storages may have areas in a memory successively allocated thereto for storing the projection data whenever storage of the projection data is switched between the first and second storages, such that new areas are allocated to the first and second storages without overlapping an area already allocated to the second storage.
With this construction, projection data may be stored continually in the memory while saving at least the projection data stored in the second storage set to the memory.
In the above construction, a new area may be allocated to the second storage when storage of the projection data is switched from the first storage to the second storage, without overlapping an area allocated to the first storage before storage of the projection data is switched.
This construction allows saving of also the latest projection data stored in the portions of the memory allocated to the first storage. When it is desired to save the latest projection data stored in the first storage, the projection data need not be copied from the first storage to the second storage.
In the above construction, a new area may be allocated to the second storage when storage of the projection data is switched from the first storage to the second storage, to overlap an area allocated to the first storage before storage of the projection data is switched.
With this construction, the portions of the memory used as the first storage may be allocated efficiently to the second storage. Thus, an efficient use is made of the memory providing the first and second storages.
In the above construction, a new area may be allocated to the second storage when storage of the projection data is switched from the first storage to the second storage, with a selection made whether the new area should overlap or should be separate from an area allocated to the first storage before storage of the projection data is switched.
This construction allows the latest projection data stored in the portions of memory allocated to the first storage to be saved or discarded as desired. Thus, only necessary projection data may be saved while making efficient use of the memory providing the first and second storages.
In the above X-ray CT apparatus, images may be reconstructed from the projection data stored in the second storage and displayed on the display device after CT fluoroscopy.
With this construction, high quality images reconstructed from the projection data stored in the second storage may be confirmed on the display device immediately after CT fluoroscopy.